CHANGES IN RESPIRATORY MECHANICS IN MYASTHENIA GRAVIS WITH THERAPY

Abstract

Abstract. The effects of anticholinesterase, and subsequently sympathomimetics, on respiratory mechanics have been studied in ten patients, with moderate to rather severe myasthenia gravis, one of them being studied separately. A systematic increase in lung resistance was first shown to occur. The latter seemed to increase with larger doses of anticholinesterase while sympathomimetics gave a return to the initial values. Subsequently, increases in the maximum respiratory pressures (P 1 max and P E max), the static lung volumes with maximum inspiration included and all dynamic lung volumes were found. Considering the basic disease, the increases in respiratory forces were thought of causal importance for the increased lung volumes. Using the functional relationships found in the untreated state this could also be shown for the change in the vital capacity. For the dynamic lung volumes the changes in the forced inspiratory (FIV 1.0 ) and expiratory volume (FEV 1.0 ) were primarily mediated by this dimensional effect i.e. increase in vital capacity. The increase in maximum voluntary ventilation (MVV) was, however, dependent on the dimensional effect to about the same extent as on a more direct muscular effect on the ventilatory forces. These changes are in principle in agreement with existing conditions in healthy controls. The successive deterioration of the disease in one patient and varying degrees of medication made it possible to instructively illustrate both these mechanical interrelationships and important implications for a simultaneous slight airway obstructive disease when administering anticholinesterase. Thus, the present results indicate that the clinical improvement in myasthenia gravis reported in other studies after ephedrine medication may to a significant extent be explained by its sympathomimetic action on the smooth bronchial muscles.